An organic EL device using an organic substance has been used as the flat luminous body of a wall hanging television or as a light source for, for example, the backlight of a display, and has been vigorously developed.
The electroluminescence phenomenon of an organic material was observed in an anthracene single crystal by Pope et al. in 1963 (J. Chem. Phys. 38 (1963) 2042). In 1965, Helfinch and Schneider succeeded in observing relatively strong injection-type EL by means of a solution electrode system having good injection efficiency (Phys. Rev. Lett. 14 (1965) 229). As reported since then, research has been conducted on the formation of an organic light-emitting substance by means of a conjugate organic host substance and a conjugate organic activator having a fused benzene ring. Examples of the organic host substance include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples of the activator include anthracene, tetracene, and pentacene. However, each of those organic light-emitting substances is present in the form of a single layer having a thickness in excess of 1 μm, so a high electric field is needed to cause such substance to emit light. Therefore, research on a thin film device by means of a vacuum deposition method has been conducted (for example, Thin Solid Films 94 (1982) 171). A reduction in thickness has been effective in reducing a driving voltage, but has not attained a device having luminance high enough to be put into practical use.
In view of the foregoing, Tang et al. have devised an EL device obtained by laminating two extremely thin layers (a hole-transporting layer and a light-emitting layer) between an anode and a cathode by means of a vacuum deposition, and have realized high luminance at a low driving voltage (Non-Patent Document 1 or Patent Document 1). After that, as a result of ten and several years of development of an organic compound to be used in each of the hole-transporting layer and the light-emitting layer, a lifetime and luminous efficiency at practical levels have been achieved. As a result, an organic EL device has started to be practically used in, for example, the display portion of a car stereo or of a portable phone.
However, the organic EL device has, for example, practically insufficient emission luminous and practically insufficient durability against the deterioration of the device with time due to long-term use, so the additional improvement of the device has been requested. In particular, when one attempts to apply the device to a full-color display or the like, the device is requested to achieve a half life of several thousand hours or longer at a high luminance of 300 cd/m2 or more for each of R, G, and B colors. It is difficult to achieve such half life particularly in the case of blue light emission. Blue light emission requires a large energy gap of the light-emitting layer (2.8 eV or more). In addition, an energy barrier upon hole injection between the hole-transporting layer and the light-emitting layer is large. Accordingly, the intensity of an electric field to be applied to an interface between the hole-transporting layer and the light-emitting layer is large. Therefore, the conventional hole-transporting layer has not allowed stable hole injection, so the improvement of the layer has been requested.
In addition, it has been pointed out that the storage performance of the organic EL device at a high temperature equal to or higher than 100° C. is problematic on the precondition that the device is mounted on a vehicle. At this time as well, it has been pointed out that the glass transition temperature of the conventional hole-transporting layer is low. One has attempted to increase the glass transition temperature to 100° C. or higher to cope with the problem. However, this approach has been still insufficient to realize good storage performance at a high temperature. Furthermore, there has been a problem in that an exciplex occurs as an interaction between the hole-transporting layer and the light-emitting layer to deteriorate the luminance of the device.    Patent Document 1: U.S. Pat. No. 4,356,429    Non-Patent Document 1: Appl. Phys. Lett. 51 (1987) 913